Method of making an antireflective silica coating, resulting product, and photovoltaic device comprising same

ABSTRACT

A low-index silica coating may be made by forming silica sol comprising a silane and/or a colloidal silica. The silica precursor may be deposited on a substrate (e.g., glass substrate) to form a coating layer. The coating layer may then be cured and/or fired using temperature(s) of from about 550 to 700° C. A capping layer composition comprising an antifog composition including a siloxane and/or hydrofluororether may be formed, deposited on the coating layer, then cured and/or fired to form a capping layer The capping layer improves the durability of the coating. The low-index silica based coating may be used as an antireflective (AR) film on a front glass substrate of a photovoltaic device (e.g., solar cell) or any other suitable application in certain example instances.

Certain example embodiments of this invention relate to a method ofmaking a low-index silica coating having an overcoat or capping layer.In certain example embodiments, the coating may comprise anantireflective (AR) coating supported by a glass substrate for use in aphotovoltaic device or the like in certain example embodiments. Thecapping or overcoat layer may include siloxane(s) and/orhydrofluoroether(s).

BACKGROUND OF THE INVENTION

Glass is desirable for numerous properties and applications, includingoptical clarity and overall visual appearance. For some exampleapplications, certain optical properties (e.g., light transmission,reflection and/or absorption) are desired to be optimized. For example,in certain example instances, reduction of light reflection from thesurface of a glass substrate may be desirable for storefront windows,display cases, photovoltaic devices (e.g., solar cells), picture frames,other types of windows, greenhouses, and so forth.

Photovoltaic devices such as solar cells (and modules therefor) areknown in the art. Glass is an integral part of most common commercialphotovoltaic modules, including both crystalline and thin film types. Asolar cell/module may include, for example, a photoelectric transferfilm made up of one or more layers located between a pair of substrates.One or more of the substrates may be of glass, and the photoelectrictransfer film (typically semiconductor) is for converting solar energyto electricity. Example solar cells are disclosed in U.S. Pat. Nos.4,510,344, 4,806,436, 6,506,622, and 5,977,477, the disclosures of whichare hereby incorporated herein by reference.

Substrate(s) in a solar cell/module are sometimes made of glass.Incoming radiation passes through the incident glass substrate of thesolar cell before reaching the active layer(s) (e.g., photoelectrictransfer film such as a semiconductor) of the solar cell. Radiation thatis reflected by the incident glass substrate does not make its way intothe active layer(s) of the solar cell, thereby resulting in a lessefficient solar cell. In other words, it would be desirable to decreasethe amount of radiation that is reflected by the incident substrate,thereby increasing the amount of radiation that makes its way to theactive layer(s) of the solar cell. In particular, the power output of asolar cell or photovoltaic (PV) module may be dependant upon the amountof light, or number of photons, within a specific range of the solarspectrum that pass through the incident glass substrate and reach thephotovoltaic semiconductor.

Because the power output of the module may depend upon the amount oflight within the solar spectrum that passes through the glass andreaches the PV semiconductor, certain attempts have been made in anattempt to boost overall solar transmission through the glass used in PVmodules. One attempt is the use of iron-free or “clear” glass, which mayincrease the amount of solar light transmission when compared to regularfloat glass, through absorption minimization.

Another attempt to boost overall solar transmission involves the use ofporous silica as an antireflective coating on glass substrate. But theenvironmental durability of AR coatings derived from porous silica maybe an issue if the coating is cast on the glass substrate at highhumidity and/or temperature. When water contacts glass, an ion exchangeprocess may begin, in which sodium ions in the glass are displaced byhydrogen ions from the water. The immediate outcome can be thehydration, or dealkalization, of the glass and depletion of the hydrogenions from the water. This process can be accompanied by a shift in theaqueous equilibrium to produce more H⁺ and OH⁻ ions (i.e., H₂O→H⁺+OH⁻).

This ion exchange process may be temperature and humidity dependent. Ifthis process occurs over a sufficiently long period of time, there maybe degradation in the surface quality due to alkali attack on the glasssilicate network. This degradation may manifest itself in one or moreforms, such as: (1) A distinctive milky white haze, which may be seen inall the glass (with or without a coating) after reaction in highhumidity and/or freezing conditions; and/or (2) Microscopic pitting ofglass occurs, wherein the pits may develop into tiny crevices that growand eventually undercut the surface, forming islands of glass which canexfoliate from the underlying bulk material.

These defects may lead to a reduction in transmissivity of anantireflecting coating after the high humidity and temperaturevariation. Therefore, there may be a need to minimize the reduction intransmission to maintain the performance of the antireflecting coatingsin the environmental conditions such as high humidity and temperatureconditions.

In one aspect of the present invention, there is a capping layer onantireflecting coatings that may minimizes the direct contact of waterto the coating and substrate. It may lead to an environmentally durableAR coating. Accordingly, in one embodiment, this invention relates touse of a capping layer, such as, an antifog coating on a temperable ARcoating on glass substrate, and possibly a minimization of reduction intransmittance after the exposure of high humidity and temperatureconditions (such as, for example, thermal and dampness/wetness testing).

BRIEF SUMMARY OF EXAMPLE EMBODIMENTS OF THE INVENTION

In certain example embodiments of this invention, there is provided amethod of making a low-index silica based coating, the methodcomprising: forming a silica precursor comprising a silica solcomprising a silane and/or a colloidal silica; depositing the silicaprecursor on a glass substrate to form a coating layer; curing and/orfiring the coating layer in an oven at a temperature of from about 550to 700° C. for a duration of from about 1 to 10 minutes; depositing acapping layer composition on the coating layer; wherein the cappinglayer composition comprises an antifog composition including ahydrofluoroether and curing and/or firing the surface treatmentcomposition to form a capping layer. The method may result in a coatinghaving improved durability after exposure to high humidity and hightemperature and/or low humidity and low temperature when compared to acoating not including the capping layer.

In certain embodiments, deposition may occur using at least one of thefollowing: flow-coating, spin-coating, roller-coating, andspray-coating. In preferred embodiments, the antifog compositioncomprises a siloxane. In other preferred embodiments, the antifogcomposition comprises a hydrofluoroether and may corresponds to theformula R_(f)(OR_(h))_(n), wherein R_(f) is a perfluorinated alkylgroup, wherein R_(h) is an alkyl group, and n is a number ranging from 1to 3, and wherein a number of carbon atoms contained in R_(f) is greaterthan a total number of carbon atoms contained in all R_(h) groups. Incertain embodiments, R_(f) comprises between 2 and 8 carbon atoms and isa linear or branched perfluoroalkyl group.

In certain embodiments, there is a method of making a photovoltaicdevice comprising a photoelectric transfer film, at least one electrode,and the low-index coating, wherein the method of making the photovoltaicdevice comprises making a low-index coating comprising a capping layerincluding a siloxane and/or hydrofluoroether, and wherein the low-indexcoating is provided on a light incident side of a front glass substrateof the photovoltaic device.

In certain embodiments, there is a method of making a photovoltaicdevice including a low-index silica based coating used in anantireflective coating, the method comprising: forming a silicaprecursor comprising a silica sol comprising a silane and/or a colloidalsilica; depositing the silica precursor on a glass substrate to form acoating layer; curing and/or firing the coating layer in an oven at atemperature of from about 550 to 700° C. for a duration of from about 1to 10 minutes; depositing a capping layer composition on the coatinglayer; wherein the capping layer composition comprises an antifogcomposition including a siloxane and/or hydrofluoroether; curing and/orfiring the surface treatment composition to form a capping layer; themethod resulting in a coating having improved durability after exposureto high humidity and high temperature and/or low humidity and lowtemperature when compared to a coating not including the capping layer;using the glass substrate with the low-index silica based coatingthereon as a front glass substrate of the photovoltaic device so thatthe low-index silica based coating is provided on a light incident sideof the glass substrate.

In certain embodiments, there is a photovoltaic device comprising: aphotovoltaic film, and at least a glass substrate on a light incidentside of the photovoltaic film; an antireflection coating provided on theglass substrate; wherein the antireflection coating comprises at least alayer provided directly on and contacting the glass substrate, the layerproduced using a method comprising the steps of: forming a silicaprecursor comprising a silica sol comprising a silane and/or a colloidalsilica; depositing the silica precursor on a glass substrate to form acoating layer; curing and/or firing the coating layer in an oven at atemperature of from about 550 to 700° C. for a duration of from about 1to 10 minutes; depositing a capping layer composition on the coatinglayer, wherein the capping layer composition comprises an antifogcomposition including a hydrofluoroether; curing and/or firing thesurface treatment composition to form a capping layer; wherein the layerhas an improved durability after exposure to high humidity and hightemperature and/or low humidity and low temperature when compared to alayer not including the capping layer.

In certain embodiments, there is a coated article comprising: a glasssubstrate; an antireflection coating provided on the glass substrate;wherein the antireflection coating comprises at least a layer provideddirectly on and contacting the glass substrate, the layer produced usinga method comprising the steps of: forming a silica precursor comprisinga silica sol comprising a silane and/or a colloidal silica; depositingthe silica precursor on a glass substrate to form a coating layer;curing and/or firing the coating layer in an oven at a temperature offrom about 550 to 700° C. for a duration of from about 1 to 10 minutes;depositing a capping layer composition on the coating layer, wherein thecapping layer composition comprises an antifog composition including asiloxane and/or hydrofluoroether; curing and/or firing the surfacetreatment composition to form a capping layer; wherein the layer has animproved durability after exposure to high humidity and high temperatureand/or low humidity and low temperature when compared to a layer notincluding the capping layer.

In certain embodiments, deposition may occur using at least one of thefollowing: flow-coating, spin-coating, roller-coating, andspray-coating.

In certain embodiments, the antifog composition comprises a siloxane.Suitable siloxanes may, for example, include hexaethylcyclotrisiloxane,hexaethyl disiloxane, 1,1,3,3,5,5-hexamethyltrisiloxane,hexamethylcyclotrisiloxane, hexavinyldisiloxane, hexaphenyldisiloxane,octaphenylcyclotetrasiloxane, hexachlorodisiloxane,dichlorooctamethyltetrasiloxane,2-methoxy(polyethyleneoxy)propyl)heptamethyl trisiloxane, 3acryloxypropyl tris trimethyl siloxysilane, methylacryloxypropylheptacyclopentyl-T8silsesquioxane,octakis(dimethylsiloxy)octaprismosilsesquioxane, andoctaviny-T8-silsesquioxane.

In certain embodiments, the antifog composition comprises ahydrofluoroether and may corresponds to the formula R_(f)(OR_(h))_(n),wherein R_(f) is a perfluorinated alkyl group, wherein R_(h) is an alkylgroup, and n is a number ranging from 1 to 3, and wherein a number ofcarbon atoms contained in R_(f) is greater than a total number of carbonatoms contained in all R_(h) groups. In certain embodiments, R_(f)comprises between 2 and 8 carbon atoms and is a linear or branchedperfluoroalkyl group.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of a coated article including anantireflective (AR) coating made in accordance with an exampleembodiment of this invention (this coated article of FIG. 1 may be usedin connection with a photovoltaic device or in any other suitableapplication in different embodiments of this invention).

FIG. 2 is a cross sectional view of a photovoltaic device that may usethe AR coating of FIG. 1.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION

Referring now more particularly to the accompanying drawings in whichlike reference numerals indicate like parts throughout the severalviews.

This invention relates to antireflective (AR) coatings that may beprovided for in coated articles used in devices such as photovoltaicdevices, storefront windows, display cases, picture frames, greenhouses,other types of windows, and the like. In certain example embodiments(e.g., in photovoltaic devices), the AR coating may be provided oneither the light incident side or the other side of a substrate (e.g.,glass substrate), such as a front glass substrate of a photovoltaicdevice. In other example embodiments, the AR coatings described hereinmay be used in the context of sport and stadium lighting (as an ARcoating on such lights), and/or street and highway lighting (as an ARcoating on such lights).

In certain example embodiments of this invention, an improvedanti-reflection (AR) coating is provided on an incident glass substrateof a solar cell or the like. This AR coating may function to reducereflection of light from the glass substrate, thereby allowing morelight within the solar spectrum to pass through the incident glasssubstrate and reach the photovoltaic semiconductor so that the solarcell can be more efficient. In other example embodiments of thisinvention, such an AR coating is used in applications other thanphotovoltaic devices (e.g., solar cells), such as in storefront windows,display cases, picture frames, greenhouse glass/windows, solariums,other types of windows, and the like. The glass substrate may be a glasssuperstrate or any other type of glass substrate in different instances.

FIG. 1 is a cross sectional view of a coated article according to anexample embodiment of this invention. The coated article of FIG. 1includes a glass substrate 1 and an AR coating 3. The AR coatingincludes a first layer 3 a and an overcoat layer 3 b.

In the FIG. 1 embodiment, the antireflective coating 3 includes firstlayer 3 a comprising a silane and/or a colloidal silica. The first layer3 a may be any suitable thickness in certain example embodiments of thisinvention. However, in certain example embodiments, the first layer 3 aof the AR coating 3 has a thickness of approximately 500 to 4000 Å afterfiring.

The AR coating 3 also includes a capping layer 3 b of or includingsiloxane(s) and/or hydrofluoroether(s), which is provided over the firstlayer 3 a in certain example embodiments of this invention as shown inFIG. 1. It is possible to form other layer(s) between layers 3 a and 3b, and/or between glass substrate 1 and layer 3 a, in different exampleembodiments of this invention.

In certain example embodiments of this invention, high transmissionlow-iron glass may be used for glass substrate 1 in order to furtherincrease the transmission of radiation (e.g., photons) to the activelayer of the solar cell or the like. For example and without limitation,the glass substrate 1 may be of any of the glasses described in any ofU.S. patent application Ser. Nos. 11/049,292 and/or 11/122,218, thedisclosures of which are hereby incorporated herein by reference.Furthermore, additional suitable glasses include, for example (i.e., andwithout limitation): standard clear glass; and/or low-iron glass, suchas Guardian's ExtraClear, UltraWhite, or Solar. No matter thecomposition of the glass substrate, certain embodiments ofanti-reflective coatings produced in accordance with the presentinvention may increase transmission of light to the active semiconductorfilm of the photovoltaic device.

Certain glasses for glass substrate 1 (which or may not be patterned indifferent instances) according to example embodiments of this inventionutilize soda-lime-silica flat glass as their base composition/glass. Inaddition to base composition/glass, a colorant portion may be providedin order to achieve a glass that is fairly clear in color and/or has ahigh visible transmission. An exemplary soda-lime-silica base glassaccording to certain embodiments of this invention, on a weightpercentage basis, includes the following basic ingredients: SiO₂, 67-75%by weight; Na₂O, 10-20% by weight; CaO, 5-15% by weight; MgO, 0-7% byweight; Al₂O₃,0-5% by weight; K₂O, 0-5% by weight; Li₂O, 0-1.5% byweight; and BaO, 0-1%, by weight.

Other minor ingredients, including various conventional refining aids,such as SO₃, carbon, and the like may also be included in the baseglass. In certain embodiments, for example, glass herein may be madefrom batch raw materials silica sand, soda ash, dolomite, limestone,with the use of sulfate salts such as salt cake (Na₂SO₄) and/or Epsomsalt (MgSO₄×7H₂O) and/or gypsum (e.g., about a 1:1 combination of any)as refining agents. In certain example embodiments, soda-lime-silicabased glasses herein include by weight from about 10-15% Na₂O and fromabout 6-17% CaO, by weight.

in addition to the base glass above, in making glass according tocertain example embodiments of the instant invention the glass batchincludes materials (including colorants and/or oxidizers) which causethe resulting glass to be fairly neutral in color (slightly yellow incertain example embodiments, indicated by a positive b* value) and/orhave a high visible light transmission. These materials may either bepresent in the raw materials (e.g., small amounts of iron), or may beadded to the base glass materials in the batch (e.g., cerium, erbiumand/or the like). In certain example embodiments of this invention, theresulting glass has visible transmission of at least 75%, morepreferably at least 80%, even more preferably of at least 85%, and mostpreferably of at least about 90% (Lt D65). In certain examplenon-limiting instances, such high transmissions may be achieved at areference glass thickness of about 3 to 4 mm. In certain embodiments ofthis invention, in addition to the base glass, the glass and/or glassbatch comprises or consists essentially of materials as set forth inTable 1 below (in terms of weight percentage of the total glasscomposition):

TABLE 1 Example Additional Materials In Glass Ingredient General (Wt. %)More Preferred Most Preferred total iron (expressed 0.001-0.06%   0.005-0.04% 0.01-0.03% as Fe₂O₃): cerium oxide: 0-0.30%  0.01-0.12%0.01-0.07% TiO₂ 0-1.0%  0.005-0.1%  0.01-0.04% Erbium oxide: 0.05 to0.5% 0.1 to 0.5% 0.1 to 0.35%

In certain example embodiments, the total iron content of the glass ismore preferably from 0.01 to 0.06%, more preferably from 0.01 to 0.04%,and most preferably from 0.01 to 0.03%. In certain example embodimentsof this invention, the colorant portion is substantially free of othercolorants (other than potentially trace amounts). However, it should beappreciated that amounts of other materials (e.g., refining aids,melting aids, colorants and/or impurities) may be present in the glassin certain other embodiments of this invention without taking away fromthe purpose(s) and/or goal(s) of the instant invention. For instance, incertain example embodiments of this invention, the glass composition issubstantially free of, or free of, one, two, three, four or all of:erbium oxide, nickel oxide, cobalt oxide, neodymium oxide, chromiumoxide, and selenium. The phrase “substantially free” means no more than2 ppm and possibly as low as 0 ppm of the element or material. It isnoted that while the presence of cerium oxide is preferred in manyembodiments of this invention, it is not required in all embodiments andindeed is intentionally omitted in many instances. However, in certainexample embodiments of this invention, small amounts of erbium oxide maybe added to the glass in the colorant portion (e.g., from about 0.1 to0.5% erbium oxide).

The total amount of iron present in the glass batch and in the resultingglass, i.e., in the colorant portion thereof, is expressed herein interms of Fe₂O₃ in accordance with standard practice. This, however, doesnot imply that all iron is actually in the form of Fe₂O₃ (see discussionabove in this regard). Likewise, the amount of iron in the ferrous state(Fe⁺²) is reported herein as FeO, even though all ferrous state iron inthe glass batch or glass may not be in the form of FeO. As mentionedabove, iron in the ferrous state (Fe²⁺; FeO) is a blue-green colorant,while iron in the ferric state (Fe³⁺) is a yellow-green colorant; andthe blue-green colorant of ferrous iron is of particular concern, sinceas a strong colorant it introduces significant color into the glasswhich can sometimes be undesirable when seeking to achieve a neutral orclear color.

It is noted that the light-incident surface of the glass substrate 1 maybe flat or patterned in different example embodiments of this invention.

FIG. 2 is a cross-sectional view of a photovoltaic device (e.g., solarcell), for converting light to electricity, according to an exampleembodiment of this invention. The solar cell of FIG. 2 uses the ARcoating 3 and glass substrate 1 shown in FIG. 1 in certain exampleembodiments of this invention. In this example embodiment, the incomingor incident light from the sun or the like is first incident on cappinglayer 3 b of the AR coating 3, passes therethrough and then throughlayer 3 a and through glass substrate 1 and front transparent electrode4 before reaching the photovoltaic semiconductor (active film) 5 of thesolar cell. Note that the solar cell may also include, but does notrequire, a reflection enhancement oxide and/or EVA film 6, and/or a backmetallic contact and/or reflector 7 as shown in example FIG. 2. Othertypes of photovoltaic devices may of course be used, and the FIG. 2device is merely provided for purposes of example and understanding. Asexplained above, the AR coating 3 reduces reflections of the incidentlight and permits more light to reach the thin film semiconductor film 5of the photovoltaic device thereby permitting the device to act moreefficiently.

While certain of the AR coatings 3 discussed above are used in thecontext of the photovoltaic devices/modules, this invention is not solimited. AR coatings according to this invention may be used in otherapplications such as for picture frames, fireplace doors, greenhouses,and the like. Also, other layer(s) may be provided on the glasssubstrate under the AR coating so that the AR coating is considered onthe glass substrate even if other layers are provided therebetween.Also, while the first layer 3 a is directly on and contacting the glasssubstrate 1 in the FIG. 1 embodiment, it is possible to provide otherlayer(s) between the glass substrate and the first layer in alternativeembodiments of this invention.

Long chain organic materials having reactive end groups based on siliconand phosphorous may form self-assembled monolayers on glass surfaces.Silanes containing short organic chains such as methyl trichlorosilanemay be used to produce monolayers of coatings (e.g., first layer 3 a) onglass surface.

Dilute solutions or dispersions of coating materials in aqueous ornon-aqueous media may be applied by any conventional wet applicationtechniques. A preferred method involves application of a dilute coatingformulation by spray process on the AR coating surface immediately afterthe coated glass emerges from a tubular furnace such as tempering line,etc. Concentration of spray coating formulation and the dwell time ofthe wet coating on the AR coating surface may be varied to get maximumpacking density of monolayers. In addition thermal energy may be appliedto further enhance coating process.

Exemplary embodiments of this invention provide a new method to producea low index silica coating for use as the AR coating 3, with appropriatelight transmission properties. Exemplary embodiments of this inventionprovide a method of making a coating containing a stabilized colloidalsilica for use in coating 3. In certain example embodiments of thisinvention, the coating may be based, at least in part, on a silica solcomprising two different silica precursors, namely (a) a stabilizedcolloidal silica including or consisting essentially of particulatesilica in a solvent and (b) a polymeric solution including or consistingessentially of silica chains.

In accordance with certain embodiments of the present invention,suitable solvents may include, for example, n-propanol, isopropanol,other well-known alcohols (e.g., ethanol), and other well-known organicsolvents (e.g., toluene).

In exemplary embodiments, silica precursor materials may be optionallycombined with solvents, anti-foaming agents, surfactants, etc., toadjust rheological characteristics and other properties as desired. In apreferred embodiment, use of reactive diluents may be used to produceformulations containing no volatile organic matter. Some embodiments maycomprise colloidal silica dispersed in monomers or organic solvents.Depending on the particular embodiment, the weight ratio of colloidalsilica and other silica precursor materials may be varied. Similarly(and depending on the embodiment), the weight percentage of solids inthe coating formulation may be varied.

Several examples were prepared, so as to illustrate exemplaryembodiments of the present invention. Although the examples describe theuse of the spin-coating method, the uncured coating may be deposited inany suitable manner, including, for example, not only by spin-coatingbut also roller-coating, spray-coating, and any other method ofdepositing the uncured coating on a substrate.

In certain exemplary embodiments, the firing may occur in an oven at atemperature ranging preferably from 550 to 700° C. (and all subrangestherebetween), more preferably from 575 to 675° C. (and all subrangestherebetween), and even more preferably from 600 to 650° C. (and allsubranges therebetween). The firing may occur for a suitable length oftime, such as between 1 and 10 minutes (and all subranges therebetween)or between 3 and 7 minutes (and all subranges therebetween).

In certain exemplary embodiments, the capping layer compositioncomprises siloxane(s) and/or hydrofluoroether(s). Suitable siloxanesmay, for example, include hexaethylcyclotrisiloxane, hexaethyldisiloxane, 1,1,3,3,5,5-hexamethyltrisiloxane,hexamethylcyclotrisiloxane, hexavinyldisiloxane, hexaphenyldisiloxane,octaphenylcyclotetrasiloxane, hexachlorodisiloxane,dichlorooctamethyltetrasiloxane,2-methoxy(polyethyleneoxy)propyl)heptamethyl trisiloxane, 3acryloxypropyl tris trimethyl siloxysilane, methylacryloxypropylheptacyclopentyl-T8silsesquioxane,octakis(dimethylsiloxy)octaprismosilsesquioxane, andoctaviny-T8-silsesquioxane.

The hydrofluoroether may correspond to the following general formula:R_(f) (OR_(h))_(n), where R_(f) is a perfluorinated alkyl group; R_(h)is an alkyl group; and n is a number ranging from 1 to 3; and where thenumber of carbon atoms contained in R_(f) is greater than the totalnumber of carbon atoms contained in all R_(h) groups. In certainpreferred embodiments, R_(f) comprises between 2 and 8 carbon atoms andis selected from the group consisting of a linear or branchedperfluoroalkyl group (such as, for example, described in PCT Pub. No.WO/1999/019707, the entirety of which is incorporated herein byreference).

In certain embodiments that include a siloxane, it may be beneficial touse a primer layer prior to application and formation of the cappinglayer. The primer layer may promote adhesion of the capping layer to thecoating. Suitable primer layers include layers comprising alcohols, suchas isopropanol, ethanol, and isobutyl alcohol, and/or well-knownsolvent(s).

In some embodiments, the resulting capping layer may vary from 2 μm to50 μm, and all subranges therebetween.

Set forth below is a description of how AR coating 3 may be madeaccording to certain example non-limiting embodiments of this invention.

Example #1

The silica sol was prepared as follows. A polymeric component of silicawas prepared by using 64% wt of n-propanol (available from ChemCentral), 24% wt of glycydoxylpropyltrimethoxysilane (glymo) (availablefrom Gelest, Inc.), 7% wt of water and 5% wt of hydrochloric acid(available from VWR International). These ingredients were used andmixed for 24 hrs. The coating solution was prepared by using 21% wt ofpolymeric solution, 7% wt colloidal silica in methyl ethyl ketonesupplied by Nissan Chemicals Inc, and 72% wt n-propanol. This wasstirred for 2 hrs to give silica sol. The final solution is referred toas a silica sol. The silica coating was fabricated using the spincoating method with 1000 rpm for 18 secs. The coating was heat treatedin furnace at 625° C. for three and a half minutes. This coating doesnot have any barrier layer. The environmental durability of the coatingwas done under following conditions for high humidity and freezing.

-   -   Ramp—Heat from room temperature (25° C.) to 85° C. @ 100 C/hr;        Bring RH up to 85%.    -   Cycle 1—Dwell @ 85° C./85% RH for 1200 minutes.    -   Ramp—Cool from 85° C. to −40° C. @ 100 C/hr; Bring RH down to        0%.    -   Cycle 2—Dwell @ −40° C./0% RH for 40 minutes.    -   Ramp—Heat from −40° C. to 85° C. @ 100 C/hr; Bring the RH up to        85%.    -   Repeat—Repeat for 10 cycles or 240 hrs.

The transmission measurements were done using PerkinElmer UV-VIS Lambda950 before and after the environmental testing. The change in % T aftertesting is shown in the Table 2, i.e., 13.39.

Example #2

In Example #2, a bottom layer was made as mentioned in Example #1 andthen followed the heat treatment. After cooling down to roomtemperature, a primer layer based on isopropyl alcohol and isobutylalcohol (commercially available as SP-22 from Exxene Corporation) wasapplied on the AR coating by flow method. The coating was dried at 120°C. for 5 minutes. The coating was cooled down to room temperature. Thenantifog coating based on modified siloxane in organic solvent (diacetonealcohol) (commercially available Exxene HCAF-424 antifog solution) wasfabricated using flow coating method. The coating was dried at 125° C.for 5 minutes. The coatings were subjected to the environmental testingas illustrated in the Example #1. Transmission was measured before andafter the environmental testing and result shown in Table 2. The changein % T after testing is 3.65.

Example #3

Example #3 is same as Example #2 except the antifog coatings are basedon a hydrofluoroether (FogTech supplied by MotoSolutions, Calif.). Theantifog coating was applied by flow coating method and dried at roomtemperature. The coatings were subjected to the environmental testing asillustrated in the Example #1. Transmission was measured before andafter the environmental testing and result is shown in Table 2. Thechange in % T after testing is 0.45.

Example #4

Example #4 is the same as Example #1 except the coating was exposed tothermal cycle (−40 to +85 C) with condensation minimization and aircirculation for 20 days per IEC 61215, which is incorporated herein byreference. Transmission was measured before and after the environmentaltesting and the result is shown in Table 2. The change in % T aftertesting is 1.31.

Example #5

Example #5 is the same as Example #2 except the antifog coating wasexposed to thermal cycle (−40 to +85 C) with condensation minimizationand air circulation for 20 days per IEC 61215. Transmission was measuredbefore and after the environmental testing and result shown in Table 2.The change in % T after testing is 0.70.

Example #6

Example #6 is the same as Example #3 except the antifog coating wasexposed to thermal cycle (−40 to +85 C) with condensation minimizationand air circulation for 20 days per IEC 61215. Transmission was measuredbefore and after the environmental testing and result shown in Table 2.The change in % T after testing is 0.03.

Example #7

Example #7 is the same as Example #1 except the coating was exposed todamp testing (85 C and 85% RH) for 40 days per IEC61215. Transmissionwas measured before and after the environmental testing and the resultis shown in Table 2. The change in % T after testing is 3.06.

Example #8

Example #8 is the same as Example #2 except the antifog coating wasexposed to damp testing (85 C and 85% RH) for 40 days per IEC61215.Transmission was measured before and after the environmental testing andthe result is shown in Table 2. The change in % T after testing is 1.78.

Example #9

Example #9 is the same as Example #3 except the antifog coating wasexposed to damp testing (85 C and 85% RH) for 40 days per IEC61215.Transmission was measured before and after the environmental testing andthe result is shown in Table 2. The change in % T after testing is 0.27.

TABLE 2 Transmission of AR coatings with and without capping layerbefore and after high humidity and freeze testing. % TransmissionExample Before Testing After Testing Change Example #1 86.96 73.57 13.39Example #2 84.54 80.89 3.65 Example #3 85.25 84.80 0.45 Example #4 86.9685.65 1.31 Example #5 86.67 85.97 0.70 Example #6 85.04 85.01 0.03Example #7 86.98 83.12 3.86 Example #8 84.68 82.91 1.78 Example #9 84.9684.69 0.27

As shown in these exemplary embodiments, the durability of temperableantireflecting coatings can be enhanced for high humidity and freezeconditions using an antifog coating as a capping layer. For instance,the change in % T after high humidity and freeze condition may become 4%if the modified siloxane based antifog is used as capping layer on ARcoating in comparison to a 14% change without capping layer. For anotherexample, the change in % T after high humidity and freeze condition maybecome 1.5% if the hydrofluoroether based antifog is used on AR coatingin comparison to a 14% change without capping layer.

In yet another example, the durability of temperable antireflectingcoatings can be enhanced for thermal testing conditions using antifogcoating as capping layer. In a further example, the change in % T afterthermal testing may become 0.70% if the modified siloxane based antifogis used as capping layer on AR coating in comparison to a 1.31% changewithout capping layer. In another example, the change in % T afterthermal testing may become 0.03% if the hydrofluoroether based antifogis used on AR coating in comparison to a 1.31% change without cappinglayer.

In yet a further example, the durability of temperable antireflectingcoatings may be enhanced for damp testing conditions using antifogcoating as capping layer. In one more example, the change in % T afterdamp testing may become 1.78% if the modified siloxane based antifog isused as capping layer on AR coating in comparison to a 3.86% changewithout capping layer. In another example, the change in % T after damptesting may became 0.27% if the hydrofluoroether based antifog is usedon AR coating in comparison to a 3.86% change without capping layer.

All described and claimed numerical values and ranges are approximateand include at least some degree of variation.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

1-16. (canceled)
 17. A method of making a low-index silica basedcoating, the method comprising: forming a silica based precursorcomprising a silica sol comprising a silane and/or a colloidal silica;depositing the silica precursor on a glass substrate to form a coatinglayer; curing and/or firing the coating layer in an oven at atemperature of from about 550 to 700° C. for a duration of from about 1to 10 minutes; depositing a capping layer composition on the coatinglayer, wherein the capping layer composition comprises an antifogcomposition including a siloxane; curing and/or firing the surfacetreatment composition to form a capping layer; and wherein the methodresults in a coating having durability after exposure to high humidityand high temperature, and/or low humidity and low temperature, whencompared to a coating not including the capping layer, and wherein saidcapping layer is the outermost layer of the coating.
 18. The method ofclaim 17, wherein at least one of the steps of depositing comprisesflow-coating, spin-coating, roller-coating, or spray-coating.
 19. Amethod of making a photovoltaic device comprising a photoelectrictransfer film, at least one electrode, and the low-index coating,wherein the method of making the photovoltaic device comprises makingthe low-index coating according to claim 17, and wherein the low-indexcoating is provided on a light incident side of a front glass substrateof the photovoltaic device.
 20. A photovoltaic device comprising: aphotovoltaic film, and at least a glass substrate on a light incidentside of the photovoltaic film; an antireflection coating provided on theglass substrate; wherein the antireflection coating comprises at least alayer provided directly on and contacting the glass substrate, the layerproduced using a method comprising the steps of forming a silicaprecursor comprising a silica sol comprising a silane and/or a colloidalsilica; depositing the silica precursor on a glass substrate to form acoating layer; curing and/or firing the coating layer in an oven at atemperature or from about 550 to 700° C. for a duration of from about 1to 10 minutes; depositing a capping layer composition on the coatinglayer, wherein the capping layer composition comprises an antifogcomposition including a siloxane; curing and/or firing the surfacetreatment composition to form a capping layer; wherein the layer has animproved durability after exposure to high humidity and high temperatureand/or low humidity and low temperature when compared to a layer notincluding the capping layer.
 21. A coated article comprising: a glasssubstrate; an antireflection coating provided on the glass substrate;wherein the antireflection coating comprises at least a layer provideddirectly on and contacting the glass substrate, the layer produced usinga method comprising the steps of: forming a silica precursor comprisinga silica sol comprising a silane and/or a colloidal silica; depositingthe silica precursor on a glass substrate to form a coating layer;curing and/or firing the coating layer in an oven at a temperature offrom about 550 to 700° C. for a duration of from about 1 to 10 minutes;depositing a capping layer composition on the coating layer, wherein thecapping layer composition comprises an antifog composition including asiloxane; curing and/or firing the surface treatment composition to forma capping layer; wherein the layer has an improved durability afterexposure to high humidity and high temperature and/or low humidity andlow temperature when compared to a layer not including the cappinglayer.